The present disclosure relates generally to an arrangement for, and a method of, electro-optically reading targets by image capture and, more particularly, to an imaging lens assembly having a magnified entrance pupil for capturing return light from a target with an improved light collecting efficiency over an extended range of working distances in a field of view of a solid-state imager of an imaging reader, and for projecting the captured return light onto the imager during reading of the target.
Solid-state imaging systems or imaging readers have been used in many industries, such as retail, manufacturing, warehousing, distribution, postal, transportation, logistics, etc., to image various symbol targets, such as one- and two-dimensional bar code symbols to be electro-optically decoded and read by image capture. A known imaging reader includes a solid-state imager, e.g., a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, having a sensor array of photocells or light sensors that correspond to image elements or pixels over a field of view of the imager, and associated circuits for producing and processing electrical signals that are processed by a programmed microprocessor or controller into data indicative of the target being decoded and read. The imaging reader also includes an illuminating light assembly for illuminating the target, and an imaging lens assembly for capturing return light scattered and/or reflected from the illuminated target, and for projecting the captured return light onto the sensor array to capture an image of the illuminated target during an exposure time period.
A known imaging lens assembly comprises a plurality or group of lenses of different optical powers, such as a classical Cooke triplet having a center lens between a pair of side lenses, and an aperture stop located between one of the side lenses and the center lens. In order to image targets that can be located over a wide range of working distances relative to the reader, it is known to move the lens group, for example, with a voice coil motor, to automatically focus the target between a near position close to the reader and a far position further away from the reader. However, this mechanical lens movement is disadvantageous for several reasons. First, the mechanical lens movement generates vibrations which, in the case of a handheld reader, may be propagated through the reader to the user's hand, may generate dust to obscure the lenses, and may generate an objectionable, annoying, audible hum. In addition, the voice coil motor is very susceptible to hand motion, consumes electrical power, is expensive and notoriously slow, can be unreliable, occupies space, and increases the overall weight, size and complexity of the reader.
It is also known to use a variable focus liquid lens for the imaging lens assembly. The liquid lens uses a fluid to create a variable focus lens without any moving parts by controlling the curvature of an outer surface (meniscus) of the fluid. Although the liquid lens focuses faster than a lens that is mechanically moved, the liquid lens has a small entrance pupil limited by its available, small active area on the meniscus. As a result, the liquid lens has a low light collecting efficiency and requires the illuminating light assembly to emit illumination light with a higher intensity or brightness to compensate for the poor light collection, together with a concomitant consumption of extra electrical power. Bright illumination shining out of the reader can be annoying or uncomfortable to an operator, or to other persons nearby the reader. Moreover, the liquid lens by itself has a very limited optical power, thereby restricting the working distance range.
Accordingly, it would be desirable to provide a compact, lightweight and inexpensive, imaging lens assembly with an enlarged entrance pupil and an improved light collecting efficiency, which can rapidly focus on both one- and two-dimensional targets to be read by an imaging reader, especially a handheld reader useful in portable and mobile applications where size, weight and cost are at a premium, over an extended range of working distances, without requiring any annoying bright illumination to be generated or excess illumination power to be consumed.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The arrangement and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.